Since the beginning of the nineteen-nineties, a multiplicity of different standards for wireless WLAN communication networks (WLAN=Wireless Local Area Network) have been published by the Institute of Electrical and Electronics Engineers (IEEE) within the framework of the IEEE 802.11 standard family. Certain characteristics of the communication networks, such as transfer rates, frequency ranges, modulation methods, channel numbers, encryption and the like, have been authoritatively specified in these standards on the basis of rapidly advancing technological development.
In the existing standards the smallest unit of a WLAN communication system is the radio cell, in which access points can exchange data with a plurality of terminal devices. A plurality of radio cells can be interconnected by way of cable connections between the access points.
A most recent development within the IEEE 802.11 standard family, which is designated as IEEE 802.11s and is expected to be published as a currently valid standard in 2009, standardizes wireless communication between the network nodes. In IEEE 802.11s, the network nodes, called mesh points, or in abbreviated form MP (MP=Mesh Point), serve as routers for wireless data transmission, thereby creating a wireless meshed ad hoc radio network (mesh network).
Proactive, reactive or hybrid routing protocols can generally be implemented in communication networks.
In communication networks having a proactive routing protocol, data transmission paths established between source and destination network node are maintained in readiness for data transmission purposes, thereby enabling fast data exchange, but having in particular the disadvantage that resources are reserved which then are possibly not used for a data exchange. In the case of a reactive routing protocol, a data transmission path between source and destination network node is set up only when needed, which, though more advantageous in terms of the use of resources, is associated with a latency time for setting up the data transmission path.
In order to exploit the advantages of proactive and reactive routing protocols, a hybrid routing protocol having the designation HWMP (HWMP=Hybrid Wireless Mesh Protocol) is provided for a wireless communication network based on the IEEE 802.11s standard for the purpose of selecting a data transmission path between source and destination network node. In HWMP, a logical topology in the form of one or more routing trees can be mapped onto the physical topology of the network. In order to build and update a routing tree, a root MP (root network node) uses the broadcast method to send routing request messages at periodic time intervals to the other MPs, which messages are designated as “proactive path requests”, or in abbreviated form proactive PREQs (PREQ=Path Request). The MPs receive the PREQs, enter the corresponding path data in their routing tables and in this way set up a unidirectional data transmission path from the MP to the sending root MP. In order to keep the number of routing messages for building a local routing tree to a minimum, a so-called proactive PREP flag for a routing reply message PREP (PREP=Path Reply) can be deleted in the proactive PREQ, which is to say that the MPs receive the proactive PREQs, set up a forward path for data transmission from the MP to the root MP, but do not send any routing reply message (PREP) to the root MP, with the result that no reverse paths are set up for data transmission from the root MP to the MPs.
Since data streams between a root MP and the MPs of a routing tree are often bidirectional, it is possible in HWMP to send a routing reply message (PREP) from the MP to the root MP at the start of a data communication, i.e. before the first data packet is transmitted from an MP to a root MP, in order in this way to set up a unidirectional reverse path from the root MP to the MP sending the PREP.
The unidirectional data transmission paths (forward paths) from the MPs to the root MPs are periodically updated by means of the PREQs periodically transmitted by the root MPs so that the unidirectional forward paths of the routing tree can be adapted to match changed conditions in the mesh network. In particular, MPs newly added to the mesh network can be incorporated into routing trees, or data transmission paths that are no longer operational due, for example, to failure of a data link can be changed.
However, because the reverse paths from the root MPs to the MPs are not updated and so remain as they were set up before the transmitting of a first data packet of a data communication by transmission of the PREP by an MP, the situation can occur where, in the case of unchanged connectivity and changed link metrics, forward and reverse paths between a root MP and an MP are different, with the result that on the forward path the data packets take the more favorable (updated) path and on the reverse path take the less favorable (non-updated) path. If a data link in a data transmission path between a root MP and an MP fails, an alternative forward path between the root MP and the MP is set up as a result of the periodically transmitted PREQs, whereas a data transmission over the non-updated reverse path is no longer possible. In this case recourse is made in HWMP to standard mechanisms based on AODV (AODV=Ad hoc On demand Distance Vector), which is associated with a relatively high latency time until data packets start to be transmitted from the root MP to the MP.